1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, it relates to an in-plane switching mode liquid crystal display (IPS-LCD) device where brightness of black color is improved.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are being developed as the next generation of display devices because of their characteristics of light weight, thin profile, and low power consumption. In general, an LCD device is a non-emissive display device that displays images by making use of a refractive index difference through utilizing optical anisotropy properties of a liquid crystal material interposed between an array substrate and a color filter substrate. Of the different types of known liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
In a conventional LCD device, since the pixel electrodes and common electrodes are positioned on the lower and upper substrates, respectively, a longitudinal electric field is induced perpendicularly between the lower and upper substrates. The conventional LCD devices have high transmittance and high aperture ratio. However, the conventional LCD devices using the longitudinal electric field have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed.
The IPS-LCD devices typically include a lower substrate on which pixel electrodes and common electrodes are disposed. A liquid crystal layer is interposed between the upper and lower substrates. The upper substrate does not have any electrodes. A detailed explanation about the operational modes of a typical IPS-LCD panel will be provided while referring to FIG. 1.
FIG. 1 is a cross-sectional view illustrating the concept of a related art IPS-LCD panel. As shown in FIG. 1, an upper substrate 20 and a lower substrate 10 are spaced apart from each other, and a liquid crystal layer 30 is interposed therebetween. The upper substrate 20 and lower substrate 10 are often referred to as a color filter substrate and an array substrate, respectively. A common electrode 12 and a pixel electrode 14 are positioned on the lower substrate 10. The common electrode 12 and pixel electrode 14 are positioned such that they are parallel to each other. On a surface of the upper substrate 20, a color filter layer (not shown) is commonly positioned to correspond to an area between the pixel electrode 14 and the common electrode 12 of the lower substrate 10.
A voltage applied across the common electrode 12 and pixel electrode 14 produces an in-plane electric field “IF” through the liquid crystal molecules 32 of the a liquid crystal layer 30. The liquid crystal molecules 32 have a positive dielectric anisotropy, and thus the liquid crystal molecules 32 will align so as to be in parallel with the electric field “IF.” When no electric field is applied between the common electrode 12 and pixel electrode 14, i.e., an “off state”, the longitudinal axes of the liquid crystal (LC) molecules 32 are parallel to and form a definite angle with respect to the common electrode 12 and pixel electrode 14. For example, the longitudinal axes of the LC molecules 32 are in parallel with both the common electrode 12 and pixel electrode 14 during an off state. However, when a voltage is applied between the common electrode 12 and pixel electrode 14 during an “on state”, a lateral electric field “IF,” which is parallel to the surface of the lower substrate 10, occurs between the common electrode 12 and pixel electrode 14. The lateral electric field “IF” rearranges the LC molecules 32 such that the longitudinal axes of the LC molecules 32 are brought into coincident alignment with the electric field “IF.” Since the LC molecules switch directions while maintaining their longitudinal axes in a plane perpendicular to the direct viewing direction of a display, in-plane switching can permit a wide viewing angle for a display device. The viewing angles can range 80 to 85 degrees in up-and-down and left-and-right sides from a line vertical to the IPS-LCD panel, for example.
FIG. 2A is a plane view of an array substrate according to the related art IPS-LCD device and FIG. 2B is a plane view of an array substrate according to another related art IPS-LCD device. The common electrode and the pixel electrode of FIG. 2A are in a stripe pattern, and the common electrode and the pixel electrode of FIG. 2B are in a zigzag pattern. As shown in FIGS. 2A and 2B, a gate line “GL” is transversely arranged across the figures and a data line “DL” is disposed substantially perpendicular to the gate line “GL.” A common line “CL” is also transversely arranged across the figures in parallel with the gate line “GL” and is spaced apart from the gate line “GL.” A thin film transistor (TFT) “T” is disposed near the crossing of the gate and data lines “GL” and “DL” in each of the figures.
As shown in FIG. 2A, a plurality of common electrodes 40 extend from the common line “CL” and a plurality of pixel electrodes 42 is connected to the TFT “T.” The plurality of common electrodes 40 and the plurality of pixel electrodes 42 are parallel to the data line “DL” and alternate with each other. Since a liquid crystal layer is driven with a lateral electric field between the common electrodes 40 and the pixel electrodes 42 in the IPS-LCD device, the viewing angle property is improved. Moreover, the viewing angle property is further improved by adopting a multi domain structure. For example, a structure having a zigzag pattern shown in FIG. 2B has been suggested.
As shown in FIG. 2B, a plurality of common electrodes 50 alternate with a plurality of pixel electrodes 52. The plurality of common electrodes 50 and the plurality of pixel electrodes 52 have a zigzag pattern. Liquid crystal molecules between the common electrode 50 and the pixel electrode 52 are aligned along two directions with a bent portion as a border to constitute a multi domain structure. Accordingly, viewing angle is further improved. However, since directors of liquid crystal molecules are different from each other in the IPS-LCD device including a zigzag pattern, color inversion occurs and enhancement of the viewing angle is limited.
FIGS. 3A and 3B are schematic views showing re-alignment states of liquid crystal molecules in a normally black mode IPS-LCD device having a zigzag pattern according to the related art. FIG. 3A shows an “off state” alignment state when no voltage is applied and FIG. 3B shows “on state” alignment state when a voltage is applied.
The orientation direction in FIG. 3A and FIG. 3B is from 270° to 90°. As shown in FIG. 3A, a long axis “A1” of a liquid crystal molecule 60 corresponds to the orientation direction when no voltage is applied. In this case, a color shift can occur. For example, a bluish color may be observed along a short axis “A2” of the liquid crystal molecule 60, i.e., from 0° to 180°, when the color should be black. This bluish color results from a difference of retardation between the long axis “A1” and the short axis “A2” of the liquid crystal molecule 60, thus degrading the black color. However, when a voltage is applied to a two domain pixel, as shown in FIG. 3B, the liquid crystal molecules 60 are aligned symmetrically in the two domains “D1” and “D2.” Thus, the appearance of the black color, depending on the viewing angle, is improved slightly.